1. Field of the Invention
The present invention related to a resin-molded component, a method for manufacturing thereof, and a resin-molded diaphragm used in a loudspeaker.
2. Description of the Related Art
Conventionally, there has been known a diaphragm for a loudspeaker, which is composed of a resin-molded component. Hereupon, in order to expand the piston movement area, a diaphragm is required to have a large specific modulus E/ρ (E: elasticity modulus, ρ: density) and a large internal loss to obtain a flat frequency characteristic. Therefore, in the case of a diaphragm composed of a resin-molded component, in order to improve an elasticity modulus, a material in which high-modulus fiber or filler is highly filled in thermoplastic resin having comparatively large internal loss has been conventionally used through injection molding or sheet forming. However, due to the increase in the amount of those additives relative density of materials increases, which causes the rising of the specific modulus and reduces a resin streak length in injection molding to makes it difficult to be thinly loaded. Accordingly, the above described characteristics of large specific modulus and large internal loss have been limited in improvement thereof.
In order to improve the specific modulus, conventionally a method to reduce the density has been taken. It is known that in a diaphragm made of a polymeric material, chemical foaming agent is added to obtain a molded diaphragm. As a result of foaming, light weight can be realized; however, the diameter of foams becomes large to considerably decline Young's modulus and improvement in specific modulus can not be obtained. Moreover, foamed shape is not uniform so that outer appearance may be unattractive, which is one of problems. Lately, a foamed diaphragm, in which injection molding, chemical foaming, and reinforcing fiber are combined, is proposed.
On the other hand, with respect to resin-molding technology, micro cellular technology is lately known as a method for forming a pored foamed cell, in which supercritical fluid is used to uniformly disperse the foamed cells with the foaming density of 109/cm2 or more. According to the method, resin-molded products can be made light in weight without deteriorating the strength thereof.
Patent document 1 discloses the art relating to the above described micro cellular technology.
Also, Patent document 2 discloses the art relating to the above described foamed diaphragm in which injection molding, chemical foaming, and a reinforcing fiber are combined to be used.
[Patent document 1]                Japanese Translation of PCT International Application No. H6-506724        
[Patent document 2]                Japanese Published Patent Application No. H8-340594        
In a foamed diaphragm formed by combining injection molding, chemical foaming, and a reinforcing fiber, since cells are foamed longitudinally in the plane thickness direction within a skin layer of the diaphragm when injection molding is performed, effectiveness of reinforcing the skin layer is attained. However, each of the foamed cells has a large diameter such as several hundred μm due to chemical foaming, so that it is extremely difficult to control the size thereof and to obtain the state in which the whole plane is uniformly foamed. In order to compensate the above situation, a reinforcing fiber is also employed; however, the fiber does not act to reinforce the inside foaming cells and there remains limits on improvement in the specific modulus.
On the other hand, a diaphragm of sheet form obtained by means of a method employing the micro-cellular technology has been known, in which a crystalline thermoplastic resin sheet is impregnated with supercritical carbon dioxide gas to form simultaneously with pressure release a uniform micro-cellular sheet of approximately 10 μm. The above diaphragm of sheet form has uniformity compared to conventional foamed sheet and also has a superior appearance. However, since a sheet made of a single material such as polyester resin is used as an unfoamed crystalline resin, elastic modulus thereof is low compared to materials used for conventional diaphragms and the elastic modulus is further deteriorated after foaming. Accordingly, although specific gravity is small, elastic modulus is greatly declined, which becomes a problem when used as a diaphragm.
Further, an injection-molded diaphragm, to which this technology is applied, is also known. In the case of the micro-cellular technology, carbon dioxide gas is fed to the inside of a cylinder of a forming machine in a supercritical condition (at 7.4 MPa or higher pressure, at 31° C. or higher temperature) to form a solvent resin in which dissolved carbon dioxide is oversaturated, and simultaneously with the forming in a mold, pressure is released to cause minute foams. However, when for example a thin diaphragm is injection-molded, resin tends to solidify rapidly, so that it becomes extremely difficult to form a uniformly-foamed body on injection molding as proposed.
The object of the present invention is to provide a resin-molded component and a resin-molded loud-speaker diaphragm, in which the above problems are solved, light-weight is facilitated, specific modulus is maintained at a high level, and rigidity is enhanced.